Magnetomechanical electronic article surveillance marker with bias element having abrupt deactivation/magnetization characteristic

Abstract

A material used to form a biasing element for a magnetomechanical EAS marker has a coercivity that is lower than the coercivity of biasing elements used in conventional magnetomechanical markers. The marker formed with the low coercivity material can be deactivated by applying an AC magnetic field at a level that is lower than is required for deactivation of conventional markers. The marker with the low coercivity bias element can also be deactivated when at a greater distance from a deactivation device than was previously practical.

What is claimed is:  
     
       1. A marker for use in a magnetomechanical electronic article surveillance system, comprising: 
       (a) an amorphous magnetostrictive element; and  
       (b) a biasing element located adjacent said magnetostrictive element;  
       wherein said marker has a deactivation-field-dependent resonant-frequency-shift characteristic having a slope that exceeds 100 Hz/Oe and said biasing element is formed of a semi-hard magnetic material having a coercivity H c  of less than 55 Oe.  
     
     
       2. A marker according to claim  1 ; wherein said deactivation-field-dependent resonant-frequency-shift characteristic has a slope that exceeds 200 Hz/Oe. 
     
     
       3. A marker according to claim  2 ; wherein said deactivation-field-dependent resonant-frequency-shift characteristic has a slope that exceeds 400 Hz/Oe. 
     
     
       4. A marker according to claim  1 ; wherein said biasing element is formed of a semi-hard magnetic material having a coercivity Hc of less than 40 Oe. 
     
     
       5. A marker according to claim  4 ; wherein said biasing element is formed of a semi-hard magnetic material having a coercivity Hc of less than 20 Oe. 
     
     
       6. A marker according to claim  1 , wherein said biasing element essentially has a composition selected from the group consisting of: 
       Fe 77.54 Ni 19.28 Cr 0.19 Mn 0.31 Si 0.30 ;  
       Fe 80.18 Co 0.20 B 13.69 Si 5.82 Mn 0.11 ; and  
       Co 55.40 Fe 29.92 Ni 11.10 Ti 3.58    
       (all expressed in atomic percent). 
     
     
       7. A marker for use in a magnetomechanical electronic article surveillance system, comprising: 
       (a) an amorphous magnetostrictive element; and  
       (b) a biasing element located adjacent said magnetostrictive element;  
       wherein said biasing element is formed of a semi-hard magnetic material having a DC magnetization field characteristic such that a DC magnetic field Ha required to achieve saturation of said biasing element is less than 350 Oe;  
       said semi-hard magnetic material having an AC demagnetization field characteristic such that an AC demagnetization field Hmd having a peak amplitude of less than 150 Oe, when applied to said biasing element with said biasing element being in a fully magnetized condition, demagnetizes said biasing element to a level that is no more than 5% of a full magnetization level.  
     
     
       8. A marker according to claim  7 ; wherein said AC demagnetization field characteristic of said bias element is such that when said biasing element is in a fully magnetized condition and is exposed to an AC field Hms having a peak amplitude of 4 Oe, said biasing element remains magnetized at a level that is at least 95% of a full magnetization level. 
     
     
       9. A marker according to claim  8 ; wherein said DC magnetization field characteristic is such that said DC magnetic field Ha required to achieve saturation of said biasing element is less than 200 Oe. 
     
     
       10. A marker according to claim  9 ; wherein said DC magnetization field characteristic is such that said DC magnetic field Ha required to achieve saturation of said biasing element is less than 150 Oe. 
     
     
       11. A marker according to claim  10 ; wherein said DC magnetization field characteristic is such that said DC magnetic field Ha required to achieve saturation of said biasing element is less than 50 Oe. 
     
     
       12. A marker according to claim  7 , wherein said biasing element essentially has a composition selected from the group consisting of: 
       Fe 77.54 Ni 19.28 Cr 0.19 Mn 0.31 Si 0.30 ;  
       Fe 80.18 Co 0.20 B 13.69 Si 5.82 Mn 0.11 ; and  
       Co 55.40 Fe 29.92 Ni 11.10 Ti 3.58    
       (all expressed in atomic percent). 
     
     
       13. A method of activating and deactivating an EAS marker for use with a magnetomechanical EAS system, the method comprising the steps of: 
       providing an EAS marker formed of a magnetostrictive element and a biasing element mounted adjacent the magnetostrictive element, said biasing element formed of a semi-hard magnetic material having a coercivity H c  of less 55 Oe;  
       magnetizing said biasing element so that said biasing element provides a magnetic field to bias said magnetostrictive element for resonance at an operating frequency of said EAS system; and  
       deactivating said EAS marker by exposing said marker to an AC field having a peak amplitude of less than 150 Oe.  
     
     
       14. A method according to claim  13 , wherein said marker has a resonance characteristic that is substantially unchanged when said marker is exposed to an AC field having a peak amplitude of 4 Oe or less. 
     
     
       15. A method according to claim  14 , wherein said marker has a resonance characteristic that is substantially unchanged when said marker is exposed to an AC field having a peak amplitude of 20 Oe or less. 
     
     
       16. A method according to claim  14 , wherein said deactivating step is accomplished by exposing said marker to an AC field having a peak amplitude of less than 100 Oe. 
     
     
       17. A method according to claim  16 , wherein said marker has a resonance characteristic that is substantially unchanged when said marker is exposed to an AC field having a peak amplitude of 12 Oe or less. 
     
     
       18. A method according to claim  13 , wherein said magnetizing step is performed after said biasing element is mounted in said marker. 
     
     
       19. A method according to claim  13 , wherein said magnetizing step is performed before said biasing element is mounted in said marker.